Over the last few years, a number of so called Zero-Voltage-Transition (ZVT) techniques have been presented. ZVT is a special case of Zero-Voltage-Switching (ZVS) wherein the voltage across a pair of ZV switched semiconductors change equally and oppositely. In this way the voltage across one device transitions from zero to another value, while the voltage across the other device decreases from that same value to zero. In conventional single-ended converters, these devices include an active switch and a rectifier. In half and full-bridge topologies, this pair of devices may be two switches in the leg of a switching bridge, or two diodes in the leg of a bridge rectifier.
Strictly speaking then, a large number of conventional converters may be categorized as ZVT topologies. These would include quasi-square-wave converters (QSWC's), as well as many zero-voltage-switching (ZVS) half and full-bridge topologies. The term ZVT has more recently been associated with a smaller class of topologies within the larger ZVT family. These topologies have the additional characteristic of possessing a soft-switching mechanism that more closely resembles an active snubber as opposed to a resonant tank (as would be found in Quasi-resonant Converters (QRC), Quasi-square wave Converters (QSWC), Multi-Resonant Converters (MRC), and Resonant converters. Conventional soft-switching techniques place the resonant circuitry in cascade with the main power flow, thus forcing it to be rated for full power. This special class of ZVT topologies (referred to simply as ZVT converters hereafter) places the resonant (soft-switching) circuitry in parallel with the power stage.
The advantage gained by this implementation is that the additional circuitry need not be rated for the total converter output power, but rather for just a fraction of that power. During most of the switching cycle, this circuitry is inactive, and incurs no loss. Only near switching transitions does the circuitry become active, enabling the main active and passive switches to commute on and off with ZVS. The drawback to this type of conventional soft-switching technique is the usual addition of an auxiliary active switch.
To accomplish ZVS switching of the active and passive power switches the ZVT auxiliary circuitry must perform several tasks: It must first provide an auxiliary path for the current flowing through the passive power switch (or rectifying switch if synchronous rectification is being used). As long as the passive power switch is on, the active switch voltage cannot return to zero. Secondly, the auxiliary circuitry must displace the charge stored across the active and passive power switches such that the active power switch voltage decreases to zero for ZVS turn-on. In other words, the energy stored in the switch capacitance must be transferred to elsewhere in the circuit such that it is not dissipated when the power switch is turned on. Finally, the current flowing through the auxiliary circuitry must return to a negligible amount (ideally zero) so that its effect on the converter operation is minimized.
Conventional ZVT techniques may be judged according to component stresses, switching losses, range of ZVS operation, simplicity, and feasibility of implementation. Several techniques have recently been presented.
One technique proposed is shown implemented for a Boost converter (101) in FIG. 1. The auxiliary circuitry (101A) consisting of a switch S.sub.x, a diode D.sub.x, and a resonant inductor L.sub.x are high-lighted in a dashed box. The principle of operation for this converter may be simply stated: Once the active power switch S has turned off, input current I.sub.g charges the switch capacitance C.sub.s until its voltage reaches the output voltage, turning on rectifying diode D.sub.r. At the end of the switching cycle when the active power switch S is to be turned on, the auxiliary switch S.sub.x is first turned on with zero-current-switching (ZCS). A positive di/dt across the auxiliary inductor L.sub.x causes its current to increase linearly. Once the current equals that of the input inductor L.sub.i, the rectifying diode D.sub.r turns off inductor L.sub.x and capacitor C.sub.s resonate together until the voltage across the active power switch S equals zero. This switch may then be turned on with ZVS, and the negative di/dt across the auxiliary inductor L.sub.x causes its current to decrease linearly to zero. Once at zero, diode D.sub.x blocks any negative current flow, and the auxiliary switch S.sub.x may turn off with ZCS.
Although simple, the technique proposed possesses several drawbacks. First and foremost, ZVS operation of the active power switch S is possible only when the output voltage V.sub.o of the Boost converter is greater than twice the input voltage V.sub.g. Secondly, the auxiliary switch is not source common with the main power switch S. Therefore a floating gate drive or a bootstrap drive is necessary. Thirdly, the current flowing through the auxiliary circuit (101A) flows back into the input source thus effectively chopping the input current at every turn-on transition. This may be undesirable in some designs. Finally, it should be noted that the peak auxiliary current is equal to the input current I.sub.g plus an additional circulating current equal to the output voltage V.sub.o divided by the characteristic impedance ##EQU1## In off-line applications (for example Power Factor Correction), Circulating currents can easily exceed 4 Amps. Therefore, the peak auxiliary current is typically equal to the input current I.sub.g plus an additional 4 Amps.
Another conventional technique proposed appears similar to the technique discussed above. The auxiliary circuit (102A) high-lighted by dashed lines in FIG. 2 connects to the output rather than the input as in FIG. 1. In addition, a capacitor C.sub.x is added in parallel with diode D.sub.x. This circuit operates on a principle similar to the one presented hereinabove, except the voltage on the additional capacitor C.sub.x (equal to twice the output voltage V.sub.o when the active power switch is off) enables the converter (102) to achieve ZVS when the output voltage V.sub.o is below as well as above twice the input voltage V.sub.g. In comparison to the technique presented earlier, this technique possesses the additional advantage of not circulating current in the auxiliary circuit back to the input thus effectively chopping the input current. However, undesirable high peak currents and a floating drive are characteristic of this technique. In addition, the voltage stress on the auxiliary switch S.sub.x is equal to twice the output voltage V.sub.o. Finally, it should be noted that the turn-off time of the auxiliary switch S.sub.x is critical. While current flows through the parallel diode Dsx, switch S.sub.x must be turned on. This requirement also adds complexity to the control circuit (not shown).
This technique is implemented for a Boost converter (103) as shown in FIG. 3. Its operation may be explained as follows: With the input current I.sub.g flowing to the output through the rectifier diode S.sub.p the voltage across switch S.sub.a is clamped at the output voltage. To reduce the voltage across the switch to zero prior to turning the switch on, an auxiliary switch S.sub.x is turned on with zero current. Current through the auxiliary inductor L.sub.x increases linearly from zero until it reaches the input current. Rectifier diode S.sub.p turns off, and any further increase in the auxiliary inductor current must come from the switch capacitance C.sub.s. This inductor current does indeed increase due to the resonant behavior between L.sub.x and C.sub.s. As current flows through C.sub.s, the voltage across it decreases to zero. Once the voltage reaches zero, the diode D.sub.s in parallel with switch S.sub.A conducts and carries a current equal to the difference between the auxiliary inductor current and the input current. Switch S.sub.A may then be turned on. This circulating current equal to the input current plus V.sub.o /Z.sub.o ##EQU2## remains constant until auxiliary switch S.sub.x is turned off. With S.sub.x off, the auxiliary inductor current flows through diode D.sub.x2 to the output. This causes the auxiliary inductor current to decrease linearly back to zero where the series diode D.sub.x1 will block the current from flowing negatively.
An advantage of this technique is the limited voltage stress (equal to V.sub.o for the Boost converter) on the auxiliary switch S.sub.x. Nevertheless, it possesses several disadvantages: First, its peak auxiliary current stress is greater than the input current I.sub.g by an amount equal to V.sub.o /Z.sub.o ##EQU3## Secondly, the on-time of the auxiliary switch is critical to minimizing the conduction losses in the auxiliary circuitry. Since the minimum on-time for this switch S.sub.x varies over load and line, the on-time of switch S.sub.x must be set to the maximum (worst case) on-time over load and line. This means that at all other operating points, the auxiliary circuit will be dissipating more energy than necessary. To remedy this problem, a variable on-time scheme would need to be implemented which would increase complexity and cost of the control circuitry. Finally, a floating drive for the auxiliary switch S.sub.x is required in a number of topologies (For example Buck type).
Another conventional technique is shown in FIG. 4 as implemented in a Boost converter (104). Two advantages accompany this technique: First, the on-time of the auxiliary switch S.sub.x is not critical to the conduction losses in the auxiliary circuitry (104A). Due to the current transformer T.sub.x, the current through the auxiliary switch S.sub.x automatically decreases to zero once the voltage across the active power switch S has decreased to zero. This is independent of the turn-off of switch S.sub.x. Secondly, the peak current (and thus the rms current) through the auxiliary switch can be significantly decreased. It's peak value can be as small as 1/2 I.sub.g less than the peak current in other conventional converters.
In spite of these advantages, such a conventional technique has several drawbacks: In comparison to the previous mentioned conventional techniques, this ZVT technique requires more components, of which most significant is an auxiliary transformer T.sub.x. In addition, a number of topology implementations require floating drive circuitry for auxiliary switch S.sub.x. Finally, isolated topologies are not easily implemented since transformer leakage inductance between the active power switch S and the auxiliary circuitry deteriorate the ZVS operation the switch.